The forgotten grammatical category: Adjective use in agrammatic aphasia

Highlights

• The study investigates adjective use in narrative speech in agrammatic aphasia.

• Overall agrammatic speakers produce adjectives in similar proportions to controls.

• This suggests that verb deficits are not due to predication or low imageability.

• Agrammatic speakers show deficits with attribution and argument-taking adjectives.

• This extends previous findings from the verbal domain to the adjectival domain.

Abstract

Background

In contrast to nouns and verbs, the use of adjectives in agrammatic aphasia has not been systematically studied. However, because of the linguistic and psycholinguistic attributes of adjectives, some of which overlap with nouns and some with verbs, analysis of adjective production is important for testing theories of word class production deficits in agrammatism.

Aims

The objective of the current study was to compare adjective use in agrammatic and healthy individuals, focusing on three factors: overall adjective production rate, production of predicative and attributive adjectives, and production of adjectives with complex argument structure.

Method and procedures

Narratives elicited from 14 agrammatic and 14 control participants were coded for open class grammatical category production (i.e., nouns, verbs, adjectives), with each adjective also coded for its syntactic environment (attributive/predicative) and argument structure.

Outcomes and results

Overall, agrammatic speakers used adjectives in proportions similar to that of cognitively healthy speakers. However, they exhibited a greater proportion of predicative adjectives and a lesser proportion of attributive adjectives, compared to controls. Additionally, agrammatic participants produced adjectives with less complex argument structure than controls.

Conclusions

The overall normal-like frequency of adjectives produced by agrammatic speakers suggests that agrammatism does not involve an inherent difficulty with adjectives as a word class or with predication, or that it entails a deficit in processing low imageability words. However, agrammatic individuals' reduced production of attributive adjectives and adjectives with complements extends previous findings of an adjunction deficit and of impairment in complex argument structure processing, respectively, to the adjectival domain. The results suggest that these deficits are not tied to a specific grammatical category.

Introduction

Over the last decades, much research has examined how processing and production of different grammatical categories are affected in agrammatic aphasia. One prominent finding, which has been reported repeatedly, is that many individuals with agrammatism have difficulty with verb as compared to noun retrieval, both in confrontation naming and in sentence production, where main verbs are often omitted (Miceli et al., 1988, Miceli et al., 1984, Myerson and Goodglass, 1972, Semenza et al., 1997).

Interestingly, in contrast to this extensive literature, little research has focused on the third major grammatical category in human languages, i.e. adjectives. Though variable, the frequency of adjectives in different corpora is around 7% (Hudson, 1994), thus this part of speech constitutes a substantial proportion of language. The few mentions in the literature of adjective production in agrammatic aphasia are mostly anecdotal, and agrammatic patterns reported differ across studies. While some studies suggest production patterns similar to that of cognitively healthy speakers and, in some cases, even over-production of adjectives by agrammatic individuals (Benson and Ardila, 1996, Varley and Siegal, 2000), others find that agrammatic aphasia is characterized by a paucity of adjectives (Bernstein, 2010, Menn, 2004).

In the present paper we investigated adjective use in spontaneous speech in agrammatic aphasia, in order to determine whether and how it differs from adjective use in the speech of cognitively healthy speakers. In addition to being interesting in itself, the study of adjectives in aphasia can shed light on several broader questions having to do with agrammatism. In the following subsections we present these questions and discuss how adjectives can contribute to answering them.

The finding that verb production is often impaired in agrammatic aphasia has been accounted for in numerous ways. In a number of papers, Bird et al. argue that agrammatic speakers' difficulty with verbs can be reduced to a general problem with less imageable concepts (Bird et al., 2000, Bird et al., 2003). Since verbs do not refer to concrete objects, they are harder to process than nouns, which often do. The authors further suggest that some patients' verb impairment may be due to damage to functional semantic features (in the sense of Warrington & Shallice, 1984), such that actions, mainly defined by functional properties, are difficult to process. In contrast, other authors have suggested that the primary reason for verb impairment is grammatical, rather than semantic. Caramazza and Hillis (1991) and Hillis and Caramazza (1995) suggest that word forms in a specific grammatical category can be selectively impaired. In addition, Shapiro et al. (Shapiro and Caramazza, 2003, Shapiro et al., 2000) propose that verb impairments may stem from a morpho-syntactic deficit, namely difficulty with person agreement. A similar explanation is offered by Friedmann (2000), who argues that the functional heads relevant for agreement and tense inflection are missing in the syntactic structures generated by agrammatic individuals. Another account of verb deficits in agrammatism focuses on the special role of verbs in sentences, namely the fact that the verb is typically the sentential predicate, assigning thematic roles to the noun phrases accompanying it. Luria (1970) proposed that the basic problem in agrammatism is a loss of predication, namely an inability to establish a relation between a predicate and its subject. Along a similar line, the fact that verb production in agrammatism becomes progressively more damaged as the number of thematic roles of the verb increases (Kim and Thompson, 2000, Kim and Thompson, 2004, Thompson et al., 1997), coupled with the fact that nouns typically do not assign thematic roles, indicates that verb production deficits may stem from the argument structure complexity associated with verbs (see Kauschke & Stenneken, 2008).

The debate with regard to the source for the verb–noun processing dissociation has been ongoing for decades and is still not settled. One of the reasons for this is that nouns and verbs differ on numerous dimensions – imageability, agreement, predication, etc. – and thus any difference between the categories can be attributed to any of these aspects. Adjectives can shed new light on this issue, since they share certain properties with nouns, and others with verbs. Adjectives are like nouns in that they do not inflect for tense, and do not show person or gender agreement, at least in English (e.g. ‘I am/you are/he is/she is tall’). They also resemble nouns in not denoting actions (but rather properties or states), thus arguably not relying on functional semantic features. On the other hand adjectives, like verbs, prototypically function as sentential predicates, predicated over the subject. They are also verb-like in that many of them are less imageable than concrete objects (consider e.g. married, ready, sorry and jealous, among many others).

Given this, knowing whether adjective production is spared or impaired in agrammatic aphasia can provide valuable information with regard to the source of verb production deficits. Despite the potential for adjectives to inform word class production deficits in aphasia, few studies have attempted to quantify adjective production abilities in aphasic individuals. In one study Menn (1990) analyzed narratives elicited from two agrammatic English speakers, noting their proportion of adjective (A) compared to verb (V) use (i.e., the A:V ratio). In the two agrammatics' narratives, A:V was 6:26 in one, and 5:7 in the other; in two healthy controls, the proportions were 4:14 and 7:34. Notably, the small sample size in the study makes it hard to draw strong conclusions with regard to adjective production in aphasia.

In the present study, we compared the proportion of adjectives used by agrammatic patients to that used by cognitively healthy controls. If adjectives are spared in patients' speech, this means that agrammatic individuals do not have an inherent problem with predication, nor with less-imageable categories, as these two properties are shared by adjectives and verbs. Rather, the deficit in verb production would be attributed to the complex morpho-syntax of verbs, or to their action semantics. In contrast, if adjectives are impaired in agrammatism, the problem with verbs is likely to indeed be attributed to predication, or to low imageability, as these traits characterize adjectives as well.

Investigation of adjective use in agrammatism can also potentially inform models of the neural organization of grammatical category information. In the last few decades, extensive research has aimed to specify the brain regions subserving noun and verb processing (for reviews, see Crepaldi et al., 2011, Vigliocco et al., 2011). While the traditional fronto-temporal dichotomy hypothesis (Damasio & Tranel, 1993) held that verb impairments are associated with frontal lesions, whereas noun impairments arise as a result of temporal lesions, much subsequent research has undermined this proposal. In particular, as argued by Crepaldi et al. (2011), verb processing seems to be supported by a more diffuse fronto-temporo-parietal network. It is probable that different parts of this network are involved in different aspects of verb processing, i.e. action semantics, inflectional morphology, argument structure, etc. For example, action knowledge seems to be supported by fronto-parietal regions, including the motor cortex (Gerfo et al., 2008, Oliveri et al., 2004); processing of low imageability words activates the left inferior frontal gyrus (e.g. Binder et al., 2005, Fiebach and Friederici, 2004, Hoffman et al., 2010, Wise et al., 2000); and verb argument structure processes engage a frontal-temperoparietal network (see Thompson & Meltzer-Asscher, 2014, for review). Since processing of adjectives engages certain mechanisms involved in verb processing and others involved in noun processing, elucidating what brain regions are associated with adjective processing can contribute to our understanding of the functional neuroanatomical basis for noun and verb processing.

Numerous studies have established that adjuncts, compared to verb arguments, are computationally expensive for cognitively healthy individuals (e.g. Boland, 2005, Kennison, 2002, Liversedge et al., 1998, Schütze and Gibson, 1999). There is some evidence for this in agrammatic aphasic individuals as well, although mixed findings have been reported. For example, Canseco-Gonzalez, Shapiro, Zurif, and Baker (1990) observed that verb learning was disrupted in aphasic participants when adjuncts, compared to arguments, were provided in a visually-based artificial language learning task. Lee and Thompson (2011) also found, in an eye-tracking production study, that adjuncts induced greater processing cost (reflected in longer gaze durations and gaze shifts) than arguments, in both healthy and agrammatic speakers. It can be hypothesized that agrammatic speakers, whose lesions often include portions of the left inferior frontal gyrus (IFG), are impaired on adjunct attachment since this process relies heavily on the IFG, which has been associated with syntactic structure building (Friederici et al., 2003, Grewe et al., 2005 and others; see review in Friederici, 2011). However, in another study on adjunction in aphasia, Byng and Black (1989) found an advantage for production of adjuncts over arguments in three nonfluent aphasic patients. It is thus unclear to what extent agrammatic individuals experience difficulty with adjunction. Further, the aforementioned studies all examined verb phrase adjuncts (e.g. ‘The man cooked the fish in the kitchen’). Even if an adjunction deficit exists in this context, it has yet to be determined whether it extends to other structures involving adjunction.

Adjectives are relevant to this discussion since they may function both as predicates and as attributes. In their predicative use (1a), adjectives form the main predicate in the sentence, denoting some property of the sentential subject (‘Cinderella's dress’, in 1a). In English, this often means that they follow a copula. In their attributive use (1b), adjectives form part of a noun phrase, modifying its head noun. Attributive adjectives were traditionally and are still often assumed to be merged as adjuncts (2) (see e.g. Chomsky, 1981). Thus, a deficit in the production of attributive adjectives may reflect a general problem with the adjunction mechanism.

In the last couple of decades, it has been proposed by several authors that adjectives and adverbs, as well as other verb phrase “adjuncts” such as locative and temporal PPs, are not attached to the syntactic tree by adjunction, but rather occupy the specifier position of dedicated functional heads in the noun and verb phrase, respectively (Alexiadou, 1997, Cinque, 1994, Cinque, 1999, Cinque, 2004). These authors assume a hierarchy of functional heads determining the order of attributive adjectives in the noun phrase. For example, Cinque (1994) proposes the following hierarchy: possessor > cardinal > ordinal > quality > size > shape > color > nationality (e.g. ‘his three other beautiful big square red Italian vases’). Importantly, other authors believe that the adjunction analysis is the correct one, and argue that analyses such as Cinque's lead to an inflation of functional heads which duplicate independently motivated semantic distinctions (Ernst, 2002, Haider, 2000, Maienborn, 2001).

If one assumes that adjectives, adverbs and locative/temporal modifiers occupy specifiers of functional heads, rather than adjoined positions, then difficulties with these categories in aphasia could reflect not a deficit in adjunction, but rather a problem with projecting the higher functional heads in the noun and verb phrase, reminiscent of Friedmann and Grodzinsky's (1997) Tree Pruning Hypothesis with regard to the structure of CP. This analysis would further predict that if higher heads show a deficit, lower ones should show one too, but not vice versa.

Several studies have focused on predicative and attributive use of adjectives in aphasia. Menn (1990) reports predicative and attributive adjective use in the narratives of the two agrammatic speakers, mentioned above. One agrammatic speaker produced 13 predicative adjectives and only 3 attributive adjectives, but the other produced 6 predicative and 7 attributive adjectives (though the latter were not always produced correctly). Again, the small sample size limits the conclusions that can be drawn from these data.

Kolk (1978) also examined attributive adjective processing, reporting the results of a study testing three severe and three recovered agrammatic patients in a “judgment of sentence structure” task. Participants were presented with a sentence containing attributive adjectives, e.g. “Old sailors tell sad stories”, and asked to point to the two words they thought “go best together”. Presumably, these judgments reflect the ability to build correct sentence structures, e.g. [[old sailors] [tell [sad stories]]]. The results showed that whereas participants in the recovered group tended to pair the attributive adjective with the noun it modified (e.g. “old” with “sailors”), the same was not true for the severe group. The author suggested that agrammatic individuals construct impoverished syntactic structures, and that adjectives are not represented because they give rise to complex structures. While Kolk's results are indicative of difficulty with attribution, it can be argued that the meta-linguistic task chosen, namely judgment of sentence structure, may not reflect patients' actual processing. Several authors have demonstrated asymmetries between automatic processing and controlled, metalinguistic performance in aphasia, and argued that the use of meta-language may be drastically limited, even when verbal communication is relatively spared (Jakobson, 1963, Lebrun and Buyssens, 1982, Rigaudeau-McKenna, 1998, Stark, 1988).

In a more recent study, de Roo, Kolk, and Hofstede (2003) investigated predicative and attributive adjective production in Dutch-speaking agrammatic patients using a picture description task. Results showed higher proportions of attributive adjective use in controls than in the agrammatic participants. The authors proposed that attributive adjectives involve a higher processing load than predicative ones, and are thus avoided by patients.

In contrast to these results, Friederici and Frazier (1992) addressed the difference between predication and attribution in a sentence picture matching experiment, with seven Broca's aphasic individuals. Stimuli were German sentences with either small clause predication (e.g. ‘The man paints the door green’) or attribution (e.g. ‘The man paints the green door’). The authors found around 30% incorrect answers in both conditions, with no significant difference between the two.

In the current study, we aimed to evaluate the attributive and predicative usage of adjectives in agrammatic speakers. If agrammatic individuals tend to avoid using adjectives attributively, this may suggest a problem with the adjunction mechanism, manifested in NP adjunct deficits (in addition to VP adjuncts, as suggested by previous literature) or alternatively, given that the deficit obeys the hierarchical order of assumed functional heads, it may reflect pruning of the noun phrase (parallel to a hypothesized pruning of the verb phrase). If, on the other hand, attributive adjectives are unimpaired, adjunction, or the entire projection of the noun phrase, must be assumed to be spared.

Finally, adjectives provide a testing ground for the argument structure production deficits seen in many individuals with agrammatic aphasia. Research in aphasia has shown that as the number of thematic roles increases, verb production becomes more difficult for agrammatic aphasic patients. This pattern has been noted in verb naming in different languages (De Bleser and Kauschke, 2003, Kemmerer and Tranel, 2000, Kim and Thompson, 2000, Kim and Thompson, 2004, Kiss, 2000, Luzzatti et al., 2002, Thompson et al., 1997), as well as in sentence production tasks, i.e. fewer correct sentences are produced with ditransitive and transitive verbs than with intransitive verbs (Dragoy and Bastiaanse, 2010, Thompson et al., 1997). Thompson and Meltzer-Asscher (2014) propose a neurocognitive model of verb argument structure processing, arguing that this processing is subserved by three mostly left-lateralized regions: the angular gyrus (AG), the IFG, and the posterior middle and superior temporal gyri (MTG and STG, respectively). While the AG is a crucial region for representation of lexical argument structure information, the IFG is involved in syntactic structure building necessitated by the projection of arguments (as well as in syntactic movement, when it occurs), and the posterior MTG and STG play a role in the integration of the verb with its arguments.

One question that has not been completely explored in the context of argument structure deficits in agrammatism is whether the observed argument structure complexity effect is a verb-specific phenomenon, or whether it is general and independent of lexical category. Collina, Marangolo, and Tabossi (2001) and Tabossi, Collina, Caporali, Pizzioli, and Basso (2010) have shown that agrammatic Italian speakers have difficulty with argument-taking nouns (e.g. pianto ‘crying’) as opposed to non-argumental nouns (e.g. medaglia ‘medal’). However, the effect of argument structure complexity has not been investigated in the adjectival domain.

Adjectives may take different types of arguments, as exemplified in (3) (for discussion of the argument structure of adjectives, see Meltzer-Asscher, 2010, Meltzer-Asscher, 2011). In the overwhelming majority of cases, arguments are selected by predicative, rather than attributive, adjectives. Additionally, it is rarely the case that an adjective obligatorily selects a complement. Ordinarily, adjectives select optional complements, and are grammatical with no complementation (compare (3a), where the complement is obligatory, to (3b–e), where the complements are optional).(3)

1. She was fond [of animals].

2. She was worried [about the time].

3. She was nice [to her mother].

4. He was excited [that the prince was throwing a party].

5. It's wonderful [that Cinderella went to the ball].

In the current study, we examined whether the adjectives produced by agrammatic participants paralleled those produced by healthy speakers with regard to their argument structure complexity. Specifically, we investigated i) whether the predicative adjectives produced by agrammatic speakers paralleled those produced by controls in their capacity to select arguments, i.e. in the complexity of lexical information associated with them, and ii) whether those predicative adjectives that select for optional complements in fact appeared with a complement when they were used in the narratives. If patients exhibit lower argument structure complexity than controls, this suggests that the argument structure deficit observed in agrammatism is likely a general impairment, not associated with verbs only.